Thermoelectric generator



Oct. 2, 1962 J. FORMAN 3,056,912

THERMOELECTRIC GENERATOR Filed Nov. 22, 1955 2 Sheets-Sheet 1 53 52 mziszziz Z ATTORNEY Oct. 2, 1962 Filed Nov. 22, 1955 ELECTRON DISCHARGECURRENT, M|CROAMPERES ELECTRON DISCHARGE CURRENT, MICROAMPERES J. FORMAN3, THERII/IOELECTRIC GENERATOR 2 Sheets-Sheet 2 500 ESTIMATED ANODETEMPERATURE T T POWER OUTPUT, M I CROWATTS loboo LOAD RESISTANCE, OHMS*INVENTOR.

JAN FORMAN ATTORNEY 3,056,912 'IHEI-MOELECTRIC GENERATOR Jan Forman,Malvern, Pa., assignor to Burroughs Corporation, Detroit, Mich., acorporation of Michigan Filed Nov. 22, 1955, Ser. No. 548,398 14 Claims.(Cl. 322-2) This invention relates to electron discharge devices, and inparticular to such devices having an emitter electrode heated tothermionic emission temperatures and a collector electrode.

In the design of a vacuum tube device there are several factors, thechoice of which determines the perveance of the device. The perveance ofa vacuum tube for a given tube structure is an essentially constant termin an expression giving the electron current due to the electronemission from the emitter electrode, commonly referred to as thecathode, which arrives at the collector electrode, commonly referred toas the anode. The perveance of a tube is known to depend on the tubegeometry, notably the area and spacing of the emitter and collectorelectrodes, on the extent of evacuation of the tube envelope, and on theproperties of the emitter surface as determined by the nature of itscomposition and by its temperature.

With respect to the collector electrode, however, much less attentionhas been paid to the condition of its surface, since it is not intendedto function as an emitter, and since in fact the function of the tubewould be defeated in many applications if the collector surface didserve as an electron emitter. For this reason the choice of anodematerial and its surface condition has been determined primarily byconsiderations of structural convenience and Without regard to thepossible influence of the condition of the anode surface, asdistinguished from its shape, size, and placement, upon the perveance ofthe tube. These considerations apply to some degree to all electrondischarge devices without limitation to diodes. Thus electron dischargedevices may be relatively inefficient in operation due to failure toprovide an anode surface condition conducive to maximum electrondischarge current between cathode and anode.

It is an object of this invention, therefore, to provide a new andimproved electron discharge device which avoids one or more of thedisadvantages of the prior art devices.

It is another object of the invention to provide a new and improvedmethod of effecting electron discharge currents between a thermionicemitter electrode and a collector electrode.

It is a further object of the invention to provide new and improvedelectron discharge devices of increased perveance resulting from theconditioning of both the emitter and collector surfaces.

It is yet another object of the invention to provide a new and improvedelectrical generator for transducing from thermal energy, applied toelectrodes in an evacuated envelope, to electrical energy represented byan electromotive force developed between these electrodes and by thecorresponding thermionic currents passing therebetween.

In accordance with the invention, an electron discharge device, havingin an evacuated envelope an emitter electrode arranged to be heated to athermionic emission temperature and a collector electrode spacedtherefrom, comprises in association with the collector electrode, meansfor increasing the magnitude of any electron discharge currents passingthrough the device by heating the collector electrode to an elevatedtemperature above 400 C. but at least about 100 C. less than theaforementioned emitter electrode temperature. More generally expressed,the collector electrode temperature should have a value above 400 C. butless than about 90 percent, on an atent O absolute temperature scale, ofthe value of the emitter electrode temperature.

Also in accordance with the invention, the method of effecting electrondischarge currents in an evacuated envelope between an emitter electrodeheated to a thermionic emission temperature and a collector electrodecomprises applying heat to the collector electrode to raise it to anelevated temperature above 400 C. but at least about C. less than theemitter electrode temperature, whereby the magnitudes of the dischargecurrents obtainable are substantially greater than those obtainable whenthe collector electrode is subjected only to such heating as might beincidental to operation of the diode without the aforementionedapplication of heat to raise the collector electrode to such elevatedtemperature.

In accordance with a feature of the invention, an elec' trical generatorcomprises an evacuated envelope, emitter and collector electrodestherein having mutually confronting surfaces, means for heating theemitter electrode to a thermionic emission temperature and for heatingthe collector electrode to an elevated temperature above 400 C. but atleast about 100 C. less than the aforementioned emitter surfacetemperature, and circuit connections through the envelope to the emitterand collector electrodes for utilizing the electrical energy, transducedfrom the thermal energy applied to the electrodes, which is representedby an electromotive force developed between the electrodes and by thecorresponding thermionic currents passing therebetween.

In accordance with a further feature of the invention, an electrondischarge device, having a vacuum tube envelope containing two heatedelectrodes with respective, mutually confronting, closely spacedsurfaces of substantial area, comprises, on these electrodes, individualpolished surface portions of closely matching shape forming theconfronting surfaces, means for biasing these surfaces of the electrodestoward each other, and an arrangement for maintaining the relativespacing of these surface portions. This arrangement includes in one ofthe surface portions a plurality of notches, each having a predetermineddepth and each having wall portions with a predetermined mutual angle ofinclination opening outwardly toward the other of the surfaces, and alsoincludes a plurality of insulating members having arcuate portionsextending from the aforesaid other surface and seated in correspondingones of the notches at points determined by the curvature of the arcuateportions.

For a better understanding of the present invention, together with otherand further objects thereof, refer ence is had to the followingdescription taken in connection with the accompanying drawings and itsscope will be pointed out in the appended claims.

In the drawing,

FIG. 1 illustrates in sectional elevation a diode vacuum tube structureembodying the present invention and including in schematicrepresentation certain circuit components associated therewith;

FIG. 2 is a lateral sectional elevation giving a detailed view of analternative arrangement for maintaining electrode spacing in a tubestructure generally similar to that of FIG. 1;

FIG. 3 is a graph, including a family of curves for various cathodetemperatures, giving the relationship be tween the anode temperature andthe electron discharge current in a diode structure; and

FIG. 4 is a graphical representation for cold and hot anode temperatureconditions of the electron discharge current and of the correspondingpower delivered to an external load as a function of the loadresistance.

Referring now to FIG. 1 of the drawings, there may be seen in asectional view taken vertically through its center a. vacuum tubestructure comprising a vacuum tube envelope or evacuated shell includinga cylindrical glass tube 11. The open end of the glass tube 11 is sealedat the bottom thereof within a shallow cup 12 of a reinforced plastic orresin material in conventional manner. Electrode lead pins 13, 14, 15,16 and 17 protrude downward through the base 12 for eflfectingelectrical connections to the interior of the envelope. U The envelopehas been sealed at its top in conventional manner after thoroughevacuation using known techniques to obtain a high vacuum in spiteof thetendency of certain gases to be adsorbed on the internal surfaces. H v

The structures within the tube are carried by a pair of posts 18 and 13,which are supported at their lower ends asthey pass through thebase 12and at their upper ends by a mica disk 21, the peripheryof which restsagainst the inside surface of the glass tube11 near the top of the tube.The posts 18 and19 are insulated from each other electrically andthermally by the material of the base 12 and the disk 21, and the lowerends of these posts form the above-mentioned lead pins 13 and 14respectively.

An emitter electrode 22 and a collector electrode 23 are supportedwithin the tube envelope by respective bracket members 26 and 27, themember 26 being spot-welded to the post 18' and to the left end of theelectrode 22' and the member 27 being spot-welded to the post 19 and tothe right end of the electrode 23. In this way conductive connectionsare obtained from the electrode 22 through the member 26 and post 18 tothe pin 13 as well as from the electrode 23'through the member 27andpost 19 to the pin 14-. The spaced electrodes 22 and 23 have the formof metallic cylinders, each closed at one end to provide mutuallyconfrontingspaced surfaces 28 and 29 respectively, while the cylindersare open at their otherror outer ends. By appropriatedesignof thesupporting structures, and ifnecessary with the inclusion of additionalstructural members such as 26 and 27. to hold the electrodes rigidlywithin the envelope, the confronting electrode surfaces 28 and 29 can beheld in parallelism and closely spaced even though the confrontingsurfacesanay be of substantial area. Alternative arrangements providingclosely spaced electrode surfaces of large area arediscussed hereinbelowin connectionwith FIG. 2 of the drawings.

Means are provided in the FIG, 1 arrangement for heating the emitterelectrode, forexample the electrode 22 or at least its electrode surface28, toa thermionic emission temperature and for heating the collectorelectrode, for example the electrode 23 or-at least its electrodesurface 29, to an elevated temperature above 400? C. but less than atemperature of efficient thermionic emission for the collector surface29 and between about 100 C. and 400 C, less than the emitter surfacetemperature. In the illustrated embodiment this means comprises twoheater elements 31 and 32 disposed individually adjacent to and withinthe cylindrical emitter and collector electrodes 22 and 23 respectivelyand near the ends thereof whose externalsurfaces form the respectiveelectrode surfaces 28 and 29. These heater elements are supported withinthe electrode structures by respective pairs of insulated leadconductors 33, 34 and 36, 37 so that the heater circuits are insulatedelectrically from the electrode structures. Thus the heating of theinner ends of the electrodes 22 and 23, and specifically of theelectrode surfaces 28 and '29, is accomplished by radiation from theheater elements 31- and 32, which are surrounded by and supported inclosely spaced relationship to the portions of the electrode structureswhich are desired to be heated. The lead conductor 33 from the element31 and the conductor 36 from the element 32 are connected to a commonlead pin 15, while the conductors 34 and 37 from the'respective heaterelements pass to pins 16 and 17 respectively.

The aforementioned means for heating the electrodes also includes meansfor energizing the heater elements 31 and '32 to maintain the surfaces28 and 29 at the specified temperatures. The last-mentioned means'includes a source of heater energy, illustrated in the form of abattery 41 one terminal of which is connected, when the device is inuse, to the lead pin 15 and thence to one side of each of the heaterelements 31 and 32. In circuit with the battery 41 is avariable resistor42, the tap of which is shown connected to the lead pin 16 and thencethrough the conductor 34, the heater element 31 itself,

' and the conductor 33 back to the battery 41. By virture of theseconnections the emitter electrode 22 is arranged to be heated to athermionic emission temperature. The energizing means for the heaterelements further includes another variable resistor 43 in circuit withthe battery 41. The adjustable tap of the resistor 43 is connected tothe lead pin 17 and thence through the conductor 37, the heaterelement32 itself, the conductor 36, and the lead pin 15 back to the battery 41,Thus the resistor 43 is a part of a means for heating the collectorelectrode to the desired temperature mentioned hereinabove.

The circuits shown connected to the tube structure in the illustratedembodiment further include a load impedance 46 connected across the leadpins 13 and 14 so as to be in circuit with the emitter and collectorsurfaces 28 and 29. An ammeter 47, preferably of low impedance, isconnected in series in this circuit, while a voltmeter 48, preferably ofhigh impedance, is connected in shunt across the load impedance 46. I v

In the operation of the electron discharge device represented in thedrawing the resistor 42 is adjusted in a manner well known in the art toobtain the desired thermionic emission temperature in thesurface 23 ofthe emitter electrode 22. This temperature varies considerably inaccordance with the material constituting the surface 28, and thematerial of this surface may be prepared in accordance with any of thetechniques known to the art of thermionic devices. For example, thesurface 28 may be an oxide-coated cathode, in which case the electrodestructure 22 may be of any suitable alloy such as platinumiridium ornickel-platinum. Konel metal is well known for this purpose, andmetallic nickel itself has been used successfully in the structure shownin the drawing. The surface 28 may be prepared by applying a coating ofan alkaline earth carbonate such as barium carbonate and activating byheating, this forming process producing a barium oxide emitting surface.In such a case the resistor 42 preferably is adjusted to obtain athermionic emission temperature at the surface 28' of approximately700-900 C. The higher temperature involved in the activation and formingprocedure for the emissive surface may beobtained by a temporaryapplication of higher voltages between the lead pins 15 and 16.

It will be appreciated that the arrangement and operation thereof thusfar described, with the exception of the inclusion of the heater element32 Within the collector or anode electrode 23 and the connections to theresistor 43, are rather similar to the conventional diode arrangement ofthe type used, for exarnple, in circuit with a signal source, not shown,connected in series with the load impedance 46 and the ammeter 47 in theexternal circuit. With such arrangements signal currents pass throughthe ammeter 47 and the impedance 46 to develop corresponding signalpotentials across the impedance 46, as may be indicated by the voltmeter48.

Also in accordance with a recognized phenomenon, sometimes referred toas the Edison effect, the external circuit arrangement illustrated inthe drawing may be used to record the passage of small currents throughthe ammeter and the load impedance in the absence of an appliedelectrical signal. The very small energy represented by these currentsand by the corresponding voltages developed across the impedance 46 istransduced from the thermal energy introduced into the device from thesource 41 of heater energy. Thus in a specific structure involvingelectrode surfaces 23 and 29, each of approximately 0.01 square incharea and spaced about 0.01 inch from each other, the resistor 42 wasadjusted to obtain a dissipation of 4.95 watts in the heater 31,whereupon the ammeter 47 recorded a current of 60 microamperes throughthe load resistance 46 and a corresponding potential drop thereacrosswithout the appli cation of an external electromotive force to the diodeand load circuit.

Turning now to operation of the electron discharge device in accordancewith the present invention, the resistor 43 is adjusted to permit theflow of heater current through the heater winding 32 such that theaforementioned elevated temperature is obtained at the collector oranode surface 29. This surface, along with the entire electrodestructure 23, conveniently may be of metallic nickel.

While the present invention is not dependent in any way upon thevalidity of any theoretical considerations which may be developedherein, it is possible that the improvement in operation of the electrondischarge device obtainable by the heating of the collector surface 29is due to a lowering in the work function of that surface. It will beunderstood that the temperature of the surface 29 should not reach thetemperatures of substantial high thermionic emission for that surface,since it is desirable to achieve as high a net electron current flow aspossible from the emitter surface 28 to the collector surface 29.Surprisingly enough, however, the maintenance at the collector surfaceof the elevated temperatures specified herein has been found to causethe diode structure and the load impedance to carry electrical signalshaving substantially increased magnitudes under otherwise similarcircumstances. With nickel electrode structures and a barium oxidecoating on the emitter, temperatures of the collector surface within anapproximate range of 400750 C., depending on cathode temperature, havebeen found to cause very substantial increases in the perveance and inthe discharge currents delivered by the diode.

Thus with 4.95 watts dissipated in the emitter heater 31, as describedhereinabove, and an adjustment of the resistor 43 to cause thedissipation of 2.6 watts in the collector heater 32, the current throughthe ammeter 47 and the load impedance 46 increased from 60 to 505microamperes. To ascertain that a similar improvement would not havebeen obtained by dissipating the same total heating power in the emitterheater 31 alone, the resistor 43 was adjusted for zero volts across theheater 32 while the adjustment of the resistor 42 was changed toincrease the power dissipated in the heater 31 to 7.55 watts. Thiscauses the current to decrease to 40 microamperes. The fact that thelast reading was lower than the 60 microampere reading obtained withonly 4.95 watts dissipated in the emitter heater may be explained by thesensitivity of the structure to minor changes when the current and powerlevels are so low or possibly may be due to exceeding the optimumcathode emission temperature or to migration of barium from the surface23 to the surface 29 at high temperatures.

It will be appreciated that, assuming the applicability of thetheoretical discussion hereinabove, it would be possible to obtain asurface composition or structure on the collector electrode such thatits work function actually would not be decreased substantially when itis heated to the elevated temperatures specified herein, and that thispossibly might defeat the improvement expected as a result of theheating of the collector surface. Nevertheless a 5-fold to -foldimprovement in the perveance of the tube at low load impedances, and asubstantial improvement in the power output of the tube used as agenerator when the external resistive load matches the internalimpedance of the generator, easily are obtained with the nickelelectrode 23, whether or not contaminated by some barium transportedfrom the surface 28; the ordinary skill of the tube designer andmanufacturer will permit the avoidance of ineffectual collector surfaceswhich are not substantially improved by the heating of 6 the collectorsurface in accordance with the present invention.

While the FIG. 1 arrangement has been found convenient for experimentalpurposes, a great variety of diode structures and methods of heating theelectrodes obviously may be utilized in carrying out the presentinvention. For example, many arrangements of electrode heaters arepossible. The two heaters 31 and 32 even might be formed as a unitarystructure with a larger part of the heater structure adjacent to theemitter electrode and the smaller part adjacent to the collectorelectrode, so that upon application of a specified potential across theentire heater a predetermined amount of heat will be radiated to each ofthe electrode structures to obtain the temperatures, within the rangesdiscussed hereinabove, desired at the two electrode surfaces. Analternating current electrical source may be substituted, of course, forthe direct current source 41. Numerous other arrangements for heatingthe electrodes, and particularly the collector electrode, by radiationor conduction may be resorted to, and one or both electrodes even mightbe heated by radiation from a surface outside of the envelope 11.Alternatively the collector electrode, or both electrodes, might beheated by the waste heat in hot gases such as the combustion products ofa fuel-burning apparatus with suitable thermostatic control of theentrance gases to maintain the desired temperature at each electrode. Inany event, the method of effecting electron discharge currents in theevacuated envelope 11 between the emitter electrode 22 heated to athermionic emission temperature and the collector electrode 23 comprisesapplying heat to the collector electrode to raise it to theaforementioned elevated temperature, whereby the magnitudes of thedischarge currents are substantially greater than those obtainable whenthe collector electrode is not subjected to heating, except such asmight be incidental to operation of the diode without the application ofheat to raise the electrode 23 to the specified elevated temperature. Itwill be understood that the thermionic emission temperature specifiedherein and in the appended claims for the emitter surface ordinarily isnot the lowest temperature at which some emission may occur, but ratheris a temperature, Within the range of substantial emission for theemitter surface involved, which would be chosen in accordance withconventional design practice for such a surface; in other words, atemperature high enough to afford efficient cathode emission but not sohigh as to prejudice unduly the life of the heater and emitterstructures.

When the method is carried out, as indicated in the drawing, fortransducing from thermal to electrical energy, the procedure comprisesheating both electrodes of an evacuated diode structure havingrespective, mutually opposed, closely spaced emitter and collectorsurfaces to maintain the emitter electrode surface at a thermionicemission temperature while maintaining the collector electrode surfacewithin the aforementioned elevated temperature range. In such a case theelectrical load 46, in circuit with the electrodes 22 and 23, isenergized with the electrical energy which is transduced from thethermal energy applied to the electrodes and which is represented by anelectromotive force developed between the electrodes and by thecorresponding thermionic currents passing therebetween. It is noteworthythat these thermionic currents are increased substantially by themaintenance of the collector electrode at such elevated temperature, inspite of the fact that the temperature difference between the electrodeshas been decreased thereby.

Although the power obtainable from the diode structure describedhereinabove without the application of an external potential source isnot large, it nevertheless is substantial, and the thermal generator ofthis feature of the invention may be made to produce very sizable andobviously useful amounts of energy by decreasing the spacing of theelectrodes and increasing their area.

When Waste heat, for example from rocket or jet exhausts, is used forheating both electrodes, as proposed hereinabove, a generator devicewithout moving parts, of indefinitely long life, and free of maintenancedimculties is achieved. Numerous structural arrangements for realizingthe suggested electrode configuration at relatively small cost and in arelatively small volume will present themselves. For example, electrodesurfaces in the form of relatively large discs or rectangular plates maybe maintained with the desired small spacing, preferably less than 0.001inch and in the neighborhood of 0.0005 inch, by the use between theelectrode surfaces of spherical or cylindrical insulating spacers havingan accurately predetermined radius and which are seated in V-shapedgrooves machined to a predetermined V-angle in the opposed surfaces.

Referring now to FIG. 2, which illustrates a form of electrodearrangement similar to that just mentioned, there is shown in sectionalelevation the surface portions of the opposing emitter and collectorsurfaces 51 and 52, it being understood that either of these may be theemitter structure and the other the collector. One of these surfaceportions, for example 52, is provided with a plurality of V-shapedgrooves or notches 53 and 54. The depth and angular inclination of thewalls of these grooves may be determined very accurately using knowntechniques for machining such grooves. Seated in these grooves arerespective insulating members 56 and 57, which may be rods, havingcircular cross sections of closely predetermined diameter, so that theradii of curvature of the portions of the members 56 and 57 whichcontact the sides of the grooves 53 and 54 are closely con trolled. Theportions of the members 56 and 57 protruding from the grooves serve tomaintain the surface 51 at the predetermined close spacing from thesurface 52. A spring arrangement, not shown, may be provided to exert amechanical bias force urging the electrodes having the surface portions51 and 52 toward each other to maintain the members 56 and 57 in placewith approximately equal pressure on the several members. The naturalelasticity of supporting members, such as the posts 18 and 19 in theFIG. 1 arrangement, may serve to provide the necessary mechanical bias.

With respect to the tube illustrated in FIG. 1 and describedhereinabove, measurements have been made to determine the electrondischarge currents generated in this tube and passed through a suitableexternal load resistor under various conditions of cathode and anodetemperatures. Such data are recorded in the graph of FIG. 3, in whichestimated anode centigrade temperatures, Ta, are represented along theabscissa andcorresponding electron discharge currents in microamperesare represented along the ordinate. A family of six curves is shown forvarious conditions of cathode temperature, T as indicated above eachcurve in the drawing. It will be seen that high emission currents can beexpected with the oxide emitter at the usual cathode temperatures withinthe approximate range of 750 to 850 C.

In any case it will be seen from FIG. 3 that an anode temperature of atleast 400C. isadvisable for high discharge currents, and that thehighest currents are obtained when the anode temperature is about 100 C.to 400 C. less than the cathode temperature. It may be noted also that arepresentative anode surface temperature of about 825 C., correspondingto an absolute temperature of about 1100" K., would require anodetemperatures within the range of roughly 400 C. to 725 C. for maximumcurrents, corresponding to absolute temperatures of the anode surface ofbetween about 675 K. 'and 1000" K.

Accordingly, in general, anode temperatures for maximum current shouldbe less than about 90% of the cathode temperature and preferably morethan'about 60% of the cathode temperature, with temperatures expressedon an absolute scale. Similar temperature relationships,

either expressed in the usual temperature terms, such as anodetemperatures having values in Centigrade degrees equal to the specifiednumber 'of degrees less than the cathode temperature, or expressed interms of relative temperature on an absolutescal'e, may be adopted withcorresponding beneficial results for other types of emitter andcollector surfaces.

To obtain the greatest improvement in electron discharge levels incarrying out the present invention, attention should be given to thechoice of the impedance of the external load. The curves in the graph ofFIG. 4 illustrate the effect of load impedance variations when the tubearrangement described hereinabove is used as a generator with the anodecold, or unheated, and with the anode hot, or heated to the temperaturesgiving relatively high outputs.

Load resistance in ohms is represented in FIG. 4 along the abscissa on alogarithmic scale. The two solid line curves give the electron dischargecurrents in microamperes, using the ordinate scale at the left of thegraph, for the two cases of cold anode and hot anode, as indicatedadjacent to the curves. It is noted that decreasing the load resistancecauses the current to increase when the anode is heated, and that loadresistances less than about 6,000 ohms must be used to obtain relativelyhigh currents greater than those obtainable Without heating the anode.

The two dashed line power curves in FIG. 4 indicate that the tubearrangement or generator of the invention tends to have a low effectiveinternal impedance. The power output curves are identified on the graphfor the hot and cold anode cases and relate to the ordinate scale inmicrowatts at the right of the graph. These curves again show across-over at about 6,000 ohms for the two anode conditions. Poweroutput with the anode hot exceeds the highest obtainable without heatingthe anode when the load resistance has any value within the rangebetween about and 1,000 ohms, and the maximum power was recorded with aload of about 400 ohms. It appears from the curves of FIG. 4 that a fewtrials will show the best load impedance for a given tube structure andanode temperature.

While there have been described what at present are considered to bepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention. It is aimed, therefore, inthe appended claims to cover all such changes and modifications whichfall within the true spirit and scope of the invention.

What is claimed is:

1. An electrical generator, comprising: a vacuum tube envelope; emitterand collector electrodes therein having mutually confronting spacedsurfaces; two heater elements individually adjacent to said emitter andcollector electrodes; and means for energizing said heater elements tomaintain said surface of said emitter electrode at a thermionic emissiontemperature and to maintain said surface of said collector electrode atan elevated temperature above 400 C. but at least about 100 C. less thansaid emitter surface temperature.

2. An electron discharge device comprising, in combination, a vacuumtube envelope, an emitter electrode and a collector electrode in theenvelope having closely spaced apart mutually confronting emitting andcollecting surfaces, means for heating said emitter electrode to athermionic emission temperature, separate means for heating saidcollector electrode to an elevated temperature above 400 C. but at leastapproximately 100 C. less than said emitting surface temperature of theemitter electrode, and circuit connections through said envelope to'said emitter and collector electrodes.

3. An'electron discharge device comprising, in combination, an evacuatedenvelope, an emitter electrode in the envelope having a relatively flatemitter surface, a collector electrode in the envelope having arelatively flat .9 collector surface, means mounting the electrodes inthe envelope with the respective flat surfaces thereof in parallelmutually confronting relationship and spaced apart not more thanapproximately .01 inch, means for heating the emitter electrode to athermionic emission temperature, and means for heating the collectorelectrode to an elevated temperature above 400 C. but between about 100C. and 400 C. less than the emitter surface of the emitter elctrode.

4. An electron discharge device comprising, in combination, an evacuatedenvelope, an emitter electrode in the envelope having a relatively flatemitter surface, a collector electrode in the envelope having arelatively flat collector surface, means mounting the electrodes in theenvelope with the respective flat surfaces thereof in parallel mutuallyconfronting relationship and spaced apart not more than approximately.01 inch, means for heating the emitter electrode to a thermionicemission temperature, and means for heating the collector electrode to atemperature below that of the emitter electrode.

5. An electron discharge device for directly transducing thermal toelectrical energy comprising, in combination, a substantially evacuatedchamber, an emitter electrode in the chamber having a relatively flatemitter surface, a collector electrode in the chamber having arelatively flat collector surface, means mounting the electrodes in thechamber with the respective fiat surfaces thereof in parallel mutuallyconfronting relationship and spaced apart less than approximately .01inch, means for heating the emitter electrode to a thermionic emissiontemperature, and means for heating the collector electrode to atemperature cooler than that of the emitter electrode, and a circuitconnection for each electrode leading out of the chamber.

6. In an electron discharge device for directly transducing thermal toelectrical energy, an emitter electrode having a smooth electronemitting surface, a collector electrode having a smooth electroncollecting surface in close confronting relation to the emitting surfaceof the emitter electrode and spaced therefrom a distance no greater thanapproximately .001 inch, circuit leads connected to the emitter andcollector electrodes, and means for heating said emitter electrode to athermionic emission temperature and for heating said collector electrodeto a temperature cooler than that of the emitter electrode.

7. A device for converting heat directly into electricity, including, incombination, a cathode electrode having an electron emitter surface, ananode electrode having an electron collecting surface, means mountingthe cathode and anode electrodes with their respective emitting andcollecting surfaces in close mutually confronting relation and spacedapart from one another a distance no greater than approximately .0005inch, means maintaining the space between said confronting surfaces ofthe electrodes in a substantially evacuated condition, and means formaintaining the temperature of the cathode electrode at an elevatedtemperature level to caues the propagation of electrons from the emittersurface of the cathode electrode to the collector surface of the anodeelectrode and for simultaneously maintaining the temperature of theanode electrode at an elevated temperature level cooler by at leastpercent, on an absolute temperature scale, than the temperature of thecathode electrode.

8. A device for converting heat directly into electricity, comprising,in combination, a cathode electrode having an electron emitter surface,an anode electrode having an electron collecting surface, means mountingthe cathode and anode electrodes with their respective emitting andcollecting surfaces in close mutually confronting relation and spacedapart from one another a distance no greater than approximately .0005inch, means maintaining the space between said confronting surfaces ofthe electrodes substantially free from substances retarding electronpropagation thereacross, and means for maintaining the temperature ofthe cathode electrode at an elevated tempera- CAD 10 ture to cause thepropagation of electrons from the emitter surface of the cathodeelectrode to the collector surface of the anode electrode and forsimultaneously maintaining the temperature of the anode electrode at anelevated temperature cooler than the temperature of the cathodeelectrode.

9. A device for converting heat directly into electricity, comprising,in combination, a cathode electrode having an electron emitter surface,an anode electrode having an electron collecting surface, means mountingthe cathode and anode electrodes with their respective emitting andcollecting surfaces in close mutually confronting relation and spacedapart from one another a distance no greater than approximately .0005inch, means maintaining the space between said confronting surfaces ofthe electrodes substantially free from substances retarding electronpropagation thereacross, means for raising the temperature of thecathode electrode to an elevated temperature to cause the propagation ofelectrons from the emitter surface of the cathode electrode tothecollector surface of the anode electrode and for simultaneouslymaintaining the temperature of the anode electrode at an elevatedtemperature cooler than the elevated electron propagating temprature ofthe cathode electrode, and a circuit connected across said cathode andanode electrodes for utilizing the electrical energy transduced from thethermal energy applied to said electrodes and having an impedanceapproximately matching the effective impedance across the electrodes.

10. In a device for directly transducing thermal energy to electricalenergy, an emitter electrode having a smooth electron emitting surface,a collector electrode having a smooth electron collecting surface inclose confronting relation to the emitting surface of the emitterelectrode and spaced therefrom a distance no greater than approximately.0005 inch, circuit leads connected to the emitter and collectorelectrodes, and means for heating said emitter electrode to a thermionicemission temperature and for heating said collector electrode to anelevated temperature above 400 C. but between approximately 60 andpercent, on an absolute temperature scale, of said emitter electrodetemperature.

11. A device for directly transducing thermal energy to electricalenergy comprising, in combination, an emitter electrode having arelatively flat electron emitter surface, a collector electrode having arelatively flat electron collector surface, means mounting theelectrodes with their respective emitter and collector surfaces inparallel mutually confronting relationship and spaced apart less thanapproximately .001 inch, means maintaining the space between saidconfronting surfaces substantially free of substances retarding electronpropagation thereacross, means for heating the emitter electrode to atemperature providing effective thermionic emission, means for heatingthe collector electrode to a temperature cooler than that of the emitterelectrode, and a circuit connected across said electrodes for utilizingthe electrical energy transduced from the thermal energy applied to theelectrodes.

12. A device for directly transducing thermal energy to electricalenergy comprising, in combination, an emitter electrode having arelatively 'flat emitter surface, a collector electrode having arelatively flat electron collector surface and disposed with itscollector surface in mutual confronting relation to the emitter surfaceof the emitter electrode, means inter-posed between the confrontingsurfaces of the electrodes and spacing the surfaces from one another notmore than .001 inch, means for heating the emitter electrode to athermionic emission temperature, and means for heating the collectorelectrode to a temperature below that of the emitter electrode.

13. In a device for directly converting thermal energy to electricalenergy, a cathode electrode and an anode electrode, a surface portion ofthe cathode electrode being of a material having a relatively highthermionic emission property at highly elevated temperatures, a surfaceportion of'the anode electrode being of another material andelectrically conductive, means mounting the electrodes in Confrontingrelation to one another with the said surface portion of the cathodeelectrode directly opposed to said surface portion of theano'de'electrode and such that the distance separating said surfaceportions is no greater than approximately .001 inch, and meansmaintaining the space between said confronting surface portionssubstantially free from substances retarding electron propagationthereacross.

14. In a device for directly converting thermal energy to electricalenergy, a cathode electrode and an anode electrode, a surface portion ofthe cathode electrode being of a material having a relatively highthermionic emission property at highly elevated temperatures, a surfaceportion'of the anode electrode being of another material andelectrically conductive, means mounting the electrodes in "confrontingrelation to one another with the said surface portion of the cathodeelectrode directly opposed to said surface portion of the anodeelectrode and such that the 12 distance separating saidsurface portionsis no greater than approximately .0005 inch,and {means maintaining thespace between said confronting surface portions substantially free fromsubstances retarding electron propagation thereacross.

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